1. Field of the Invention
This invention relates to an exposure method and an exposure apparatus using near-field light and also to an exposure mask. More particularly, the present invention relates to an exposure method, an exposure apparatus and an exposure mask to be used for micro-processing operations exceeding the limit imposed from the wavelength of the light.
2. Related Background Art
Photolithography needs to be adapted to micro-processing operations involving ever-increasingly reduced dimensions in order to keep pace with the current technological advancement in the field of large capacity semiconductor memories and high-speed processing/highly integrated CPU processors.
Generally, the limit to micro-processing operations of a photolithography apparatus is defined by the wavelength of light used with the apparatus. Therefore, the wavelength of light that is used for photolithography apparatus has been reduced. Currently, near ultraviolet laser beams are used to realize micro-processing operations involving dimensions of about 0.1 xcexcm. However, as the trend of photolithography toward more reduced dimensions goes on for micro-processing operations, there arise problems such as availability of laser beams of shorter wavelengths and lenses that can be used for micro-processing operations that deal with dimensions smaller than 0.1 xcexcm.
Meanwhile, micro-processing apparatus realized by utilizing the configuration of a scanning near-field optical microscope (to be referred to as SNOM) hereinafter) have been proposed as means using light for performing micro-processing operations involving dimensions less than 0.1 xcexcm. For example, such an apparatus is designed to locally expose an area of a photoresist having dimensions smaller than the wavelength of light to near-field light seeping out of a micro-aperture smaller than 100 nm.
However, the micro-processing operation of a photolithography apparatus realized by utilizing the configuration of an SNOM is like that of using a single processing probe (or a few number of probes) and cannot expect a large throughput.
As an attempt to eliminate this problem, Japanese Patent Application Laid-Open No. 8-179493 discloses an arrangement using a prism relative to an optical mask and incident light is made to strike the prism with an angle that causes total reflection so as to transfer the entire pattern of the optical mask to the photoresist, using the near-field light seeping out from the total reflection plane.
On the other hand, Japanese Patent Application Laid-Open No. 11-072607 discloses an aperture pattern that can be formed on a mask. According to this patent document, when a pattern having apertures that are 150 nm to 1 xcexcm large and arranged at a pitch of 0.6 to 1.8 xcexcm, light is transmitted through the aperture array at a rate higher than the rate at which light is made to directly enter the apertures if a specific pitch is selected for the arrangement of apertures as a function of the wavelength of incident light that is between 0.5 and 1.0 xcexcm. Thus, transmission of propagated light is increased.
This phenomenon is believed to boil down to the well known phenomenon of Wood""s anomaly that, as the pitch of arrangement of bars of a diffraction grating comes close to the wavelength of light used for exposure, the reflected light becomes mostly the zero-order diffracted light. (Refer to M. Tsuruta, xe2x80x9cThe Pencil of Lightxe2x80x9d (1984) and xe2x80x9cElectromagnetic Theory of Gratingsxe2x80x9d edited by R. Petet (1980).)
In view of the fact that the intensity of light transmitted through the aperture array varies periodically as a function of the wavelength of light, it can be assumed that there exists a certain periodic condition under which a resonance will occur between the pitch of the aperture array and the wavelength of the incident light. This is a phenomenon with respect to propagated light and may involve the following problems if it is applied to light exposure.
First, the dimension of processing is limited to a size within a range between half and twice the exposure wavelength since the phenomenon is caused to occur in the resonance region. Second, the exposure wavelength cannot be selected independently of the dimension of processing since the condition of resonance must be satisfied. Third, the above phenomenon is observed in the far field region with respect to propagated light, and it cannot be immediately applied to the near field region.
While the phenomenon described above relates to propagated light, the inventors of the present invention have found that the intensity of near-field light also changes as a function of the density of apertures on a mask. Therefore, the intensity of near-field light changes when masks with different aperture densities are or a mask having areas with different aperture densities is used for exposure so that it is difficult to form a uniform pattern because the pattern is apt to become uneven due to the uneven exposure to light.
In view of the above circumstances, it is, therefore, the object of the present invention to provide an exposure method, an exposure apparatus and an exposure mask that can form a uniform pattern when using near-field light.
According to the invention, the above object is achieved by providing an exposure method using near-field light, the method comprising:
a step of controlling the position of an exposure mask to control the distance between the exposure mask and an object to be processed so as to make the object to be located in a region where near-field light is present; and
a step of exposing the object to be processed to light by controlling the intensity of near-field light as a function of the aperture density of the exposure mask.
According to the invention, there is also provided an exposure apparatus using near-field light, the apparatus comprising:
light irradiating means for irradiating light to be used for exposure;
gap controlling means for controlling the gap between an object to be processed and an exposure mask;
support means for supporting the exposure mask having at least a pair of apertures of a size smaller than the wavelength of light to be used for exposure arranged in a region of a size as large as the wavelength of light to be used for exposure; and
control means for controlling the intensity of near-field light as a function of the aperture density of the exposure mask.
According to the invention, there is also provided an exposure mask comprising at least a pair of apertures of a size smaller than the wavelength of light to be used for exposure arranged in a region of a size as large as the wavelength of light to be used for exposure.
An exposure method, an exposure apparatus and an exposure mask according to the invention can be adapted to changes in the quantity of light due to variations in the aperture density so that it is possible to produce a micro-pattern for processing operations almost free from variations depending on the aperture density.